1. Field of the Invention
The present invention relates to a semiconductor electron emission element.
2. Related Background Art
Conventional semiconductor electron emission elements, elements using avalanche amplification mechanisms are described in U.S. Pat. Nos. 4,259,678 and 4,303,930. In such a semiconductor electron emission element, p- and n-type semiconductor layers are formed on a semiconductor substrate, and cesium or the like is deposited on the surface of the n-type semiconductor layer to decrease a work function of the surface, thereby forming an electron emission portion. A reverse bias voltage is applied across two ends of a p-n junction formed by the p- and n-type semiconductor layers to cause avalanche amplification, so that electrons are converted into hot electrons. These hot electrons are emitted from the electron emission portion to the surface of the semiconductor substrate in a direction perpendicular to the surface of the semiconductor substrate.
As disclosed in Japanese Laid-Open Patent Application No. 1-220328, a Schottky barrier junction is formed by a p-type semiconductor and a metal material or a p-type semiconductor and a metal compound, and a reverse bias voltage is applied across two ends of this Schottky barrier junction to cause avalanche amplification, thereby converting electrons into hot electrons. These hot electrons are emitted from the electron emission portion to the surface of the semiconductor substrate in a direction perpendicular to the surface of the semiconductor substrate.
In each conventional semiconductor electron emission element described above, when the reverse bias voltage is applied across the two ends of the p-n or Schottky junction, avalanche breakdown occurs in the high-concentration p-type semiconductor region in which a depletion layer has the smallest width. Electrons having high energies and formed in this p-type semiconductor region are emitted from the solid surface to the outside. However, the shape of the depletion layer around the p-n or Schottky barrier junction has a radius of curvature determined by the application voltage and the carrier concentration of the p-type semiconductor. Therefore the electric field in the p-type semiconductor region becomes intense. Breakdown occurs in the p-n or Schottky barrier junction at a voltage lower than that causing the avalanche breakdown in the high-concentration p-type region, thereby degrading the characteristics of the element. It is possible to decrease the carrier concentration of the p-type semiconductor at the p-n or Schottky barrier junction and hence increase the radius of curvature around the depletion layer, thereby suppressing breakdown therein. However, the resistance value between an electrode for supplying electrons and the high-concentration p-type semiconductor region which causes the avalanche breakdown is then increased. A voltage drop and generation of Joule's heat occur in this high-resistance region. In a conventional element, in order to increase the radius of curvature around this depletion layer to prevent breakdown therein, a guard ring made of an n-type semiconductor is required. In a conventional element structure, a ring-like n-type semiconductor region is formed to have a high impurity concentration, so that a process such as ion implantation or thermal diffusion, which takes a long period of time to increase the amount of a dopant, are required. In addition, since a voltage is applied to the guard ring made of a high-concentration n-type semiconductor, an ohmic contact electrode must be formed on the guard ring so as to achieve excellent ohmic contact with the n-type semiconductor.